The present invention relates to an apparatus to detect dropout (loss of information) in reproduced signals when information stored in a predetermined format is reproduced from an information storage disk such as an optical disk or magnetic disk or the like.
A recording and reproducing apparatus for an information storage disk is disclosed in U.S. Pat. No. 4,803,677. The disclosed recording and reproducing apparatus for use in an information storage disk records information by using a light beam to form pits on the surface of the disk, and reproduces the information by using a light beam to read the pits on the surface of the disk. Information is recorded on the disk in the form of a high-frequency signal produced by frequency-modulating a video signal. The reproduction of video signals is done by demodulating the high-frequency signal reproduced from the disk. In such a reproduction procedure, dropouts may occur in the reproduced high-frequency signals due to unevenness or moisture on the disk, the attachment of dust or other foreign matter to the disk, or due to pinholes in the disk itself etc. Because of this, the reproduction device performs dropout detect processing for the reproduced high-frequency signal and performs compensation processing or the like if dropout is detected. This compensation processing, for instance, involves replacing the video signal of the detected dropout portion with a partial video signal of the corresponding portion of the previous line.
FIG. 1A is a block diagram indicating one example of a conventional dropout detector apparatus. The high-frequency signal (RF signals) reproduced from the disk is input to terminal 10. The high-frequency signal has the waveform indicated in FIG. 1B(A). A comparator 11 compares the high-frequency signal with a predetermined reference voltage and outputs a high-level voltage (hereinafter simply referred to as H-level) when the level of the high-frequency signal exceeds the reference voltage level. Accordingly, the signal output from the comparator 11 is therefore a rectangular-waveform pulse as indicated in FIG. 1B(B). The rectangular-waveform pulse is inputted to a re-triggerable mono-stable multivibrator 12. The re-triggerable mono-stable multivibrator 12 is set equal to time 1.5 T as 1.5 times the cycle T of the rectangular-waveform pulse signals from the comparator 11 on the basis of the status of the RF signal. When the re-triggerable mono-stable multivibrator 12 is triggered by said rectangular-waveform pulse signal within the time 1.5 T, it maintains an unstable status at the output H-level, and when the cycle of the rectangular-waveform pulse signal exceeds the time 1.5 T because of dropout of the RF signals (Refer to FIG. 1B(A),) it outputs a dropout detect signal of a low-level voltage (hereinafter simply referred to as L-level) as indicated in FIG. 1B(C). This dropout detect signal is outputted through terminal 13.
It is noted that the level of the high-frequency component of the RF signal is lowed if the spot diameter of the light beam is not sufficiently small when the information stored on the disk is reproduced as RF signals. When this occurs, the peaks A, B (absolute values) of the waveform of the RF signal do not exceed the reference voltage, as for example, is indicated in FIG. 1C(A), and the dropout in the RF signal is therefore detected for these portions.
The dropout detection apparatus indicated in FIG. 2(A) is proposed in order to prevent erroneous dropout detection such as the above mentioned.
This apparatus comprises a high-frequency-range compensation circuit 14 and is configured so that the RF signal is inputted to the comparator 11 after it has been processed by this high-frequency-range compensation circuit 14. Moreover, the terminals 10, 13 and the re-triggerable mono-stable multivibrator 12 have the same configuration as shown in FIG. 1A.
The high-frequency-range compensation circuit 14 amplifies high-frequency components of the RF signal so that RF signal with the waveform indicated in FIG. 1C(A) is rectified to have the waveform indicated in FIG. 1C(B). Accordingly, the new peaks A', B' corresponding to the previously mentioned peaks A, B exceed the reference voltage and erroneous dropout detection is prevented.
When the disk stores information with high recording density, it is necessary that the amount of compensation (amount of amplification) performed by the said high-frequency-range compensation circuit 14 is proportionately large.
When the amount of compensation performed by the said high-frequency-range compensation circuit 14 is large, the signal containing dropout as indicated by the dotted line in FIG. 2B(A), is compensated by the high-frequency-range compensation circuit 14 to have the waveform indicated in FIG. 2B(B). That is, the new peak C exceeding the reference voltage is formed for the dropout portion. Because of this, the output signal of the comparator 11 forms a pulse P1 corresponding to the previously mentioned peak C as indicated in FIG. 2B(C), and this creates the problem of dropout not being able to be detected even though it has occurred.